Remove some UI->FILE->UI thread hops in ExecuteCodeFunction

extensions/browser/file_reader_unittest.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
@@ skipping 17 matching lines @@
  private:
   base::MessageLoop message_loop_;
   content::TestBrowserThread file_thread_;
 };
 
 class Receiver {
  public:
   Receiver() : succeeded_(false) {
   }
 
-  FileReader::Callback NewCallback() {
+  FileReader::DoneCallback NewCallback() {
     return base::Bind(&Receiver::DidReadFile, base::Unretained(this));
   }
 
   bool succeeded() const { return succeeded_; }
   const std::string& data() const { return *data_; }
 
  private:
   void DidReadFile(bool success, std::unique_ptr<std::string> data) {
     succeeded_ = success;
     data_ = std::move(data);
@@ skipping 11 matching lines @@
   ExtensionResource resource(
       extension_id, path, base::FilePath().AppendASCII(filename));
   path = path.AppendASCII(filename);
 
   std::string file_contents;
   ASSERT_TRUE(base::ReadFileToString(path, &file_contents));
 
   Receiver receiver;
 
   scoped_refptr<FileReader> file_reader(
-      new FileReader(resource, receiver.NewCallback()));
+      new FileReader(resource, FileReader::OptionalFileThreadTaskCallback(),
+                     receiver.NewCallback()));
   file_reader->Start();
 
   base::RunLoop().Run();
 
   EXPECT_TRUE(receiver.succeeded());
   EXPECT_EQ(file_contents, receiver.data());
 }
 
 TEST_F(FileReaderTest, SmallFile) {
   RunBasicTest("smallfile");
 }
 
 TEST_F(FileReaderTest, BiggerFile) {
   RunBasicTest("bigfile");
 }
 
 TEST_F(FileReaderTest, NonExistantFile) {
   base::FilePath path;
   PathService::Get(DIR_TEST_DATA, &path);
   std::string extension_id = crx_file::id_util::GenerateId("test");
   ExtensionResource resource(extension_id, path, base::FilePath(
       FILE_PATH_LITERAL("file_that_does_not_exist")));
   path = path.AppendASCII("file_that_does_not_exist");
 
   Receiver receiver;
 
   scoped_refptr<FileReader> file_reader(
-      new FileReader(resource, receiver.NewCallback()));
+      new FileReader(resource, FileReader::OptionalFileThreadTaskCallback(),
+                     receiver.NewCallback()));
   file_reader->Start();
 
   base::RunLoop().Run();
 
   EXPECT_FALSE(receiver.succeeded());
 }
 
 }  // namespace extensions